A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use EUV radiation which is electromagnetic radiation having a wavelength selected from the range of 10-20 nm, for example the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example selected from the range of 5-10 nm, for example 6.7 nm or 6.8 nm.
Radiation may be produced in the radiation source using plasma. The plasma may be created for example by directing a laser beam at a plasma fuel, the fuel for example including or consisting of particles of a suitable material (e.g. tin), or by directing a laser at a stream of a suitable gas or vapor (acting as a plasma fuel), for example Xe gas or Li vapor. The resulting plasma emits radiation, for example EUV radiation may be collected using a collector, which receives the radiation and may focus the radiation into a beam. Such a radiation source is typically called a laser produced plasma (LPP) radiation source, for example an EUV LPP radiation source.